Data may be scrambled in various ways. A major objective of scrambling audio and video signals or other image data is security of rights related to the toll data transmission activities or the right contained in data transmitted thereby. Only the authorized persons licensed by an original author or its agency should be enabled to descramble and read the data transmitted, while it should not be made available to the rest. For protecting the rights related to data, an image data is scrambled before transmission and a key information needed for descrambling the scrambled data is transmitted, or controlled.
Japanese Patent Laid Open Publication No. H7-274166 teaches us a conventional technology. Outline of the conventional technology is described below. A principal aim of the conventional technology is to avoid any increase in the total number of bits when scrambling certain data through the variable length encoding. Therefore, instead of scrambling an input signal data, scrambling is conducted on any one bit of data after they undergo a DCT device, a sampling device, a run length encoder or a motion vector encoder, or a combination thereof. As understood also from the above, it has been a common practice to apply scrambling on compressed and encoded data. It is not desirable to conduct scrambling direct on an input linear PCM data; the data redundancy is lost by the scrambling, which eventually results in a drastic deterioration in the efficiency of compression.
Meanwhile, the recently introduced DVD and other such high density recording media have made it possible to record linear PCM audio data without having them compressed. So, in some of the AV systems that transmit both video data and audio data at the same time, compressed video data and linear PCM audio data operate in combination. The copyright security by scrambling is of course required also in such AV systems of the above operating mode. FIG. 9 shows an exemplary AV system having the above-described operating mode. In FIG. 9, numeral 100 denotes input means for linear PCM audio data, 150 is input means for linear PCM video data, 900 is compression means, 300 and 350 are scramble means, and 400 is composition means. Numeral 500 denotes separation means, 600 and 650 are descrambling means, 800 is output means for linear PCM audio data, 950 is decompression means, and 850 is output means for linear PCM video data.
Audio signal Ax is scrambled by the scramble means 300 to become scrambled audio signal Az. The scrambled audio signal Az is delivered to the composition means 400. Video signal Vx is compressed by the compression means 900 to become compressed video signal Vy, which signal is scrambled by the scramble means 350 to become compressed scrambled video signal Vz. The compressed scrambled video signal Vz is delivered to the composition means 400. The composition means 400 composes these signals to make a composite signal AV. In order to use the same means in common, in the present case, the scrambling means 300 and the scrambling means 350 follow a same scrambling method. The composite signal AV thus produced is recorded in an appropriate medium format, or transmitted.
The composite signal AV recorded, or transmitted, is received at the separation means 500, which signal is separated there into scrambled audio signal Az′ and compressed scrambled video signal Vz′. The scrambled audio signal Az′ is descrambled at the descrambling means 600 to become descrambled audio signal Ax′. The audio signal Ax′ is taken up by the output means 800 for linear PCM audio data. The compressed scrambled video signal Vz′ is descrambled at the descrambling means 650 to become compressed video signal Vy′, which video signal Vy′ is decompressed by the decompression means 950 to be restored to video signal Vx′. The video signal Vx′ is taken up by the output means 850 for linear PCM video data.
Problems in the conventional AV systems with respect to the scrambling are described in the following, referring to FIG. 10 and FIG. 11. Problems mostly occur when the signal is in a very small level or at mute, so description is made using a case at mute as an example. FIG. 10 shows an exemplary scrambling in a conventional AV system, where the input audio signal Ax is mute data. As it is exhibited in the 2S compliment, all of the bits are “0”, as shown in FIG. 10(a). Now, think of a method of main scrambling through the bit reversal using a scramble bit string generated through a formula Gx=1+X+X5+X8+/X10, for example. FIG. 12 shows an exemplary circuit structure for this case.
The scrambling string is as shown in FIG. 10(b). The scrambled output data Az, FIG. 10(c), has the same bit string as the scrambling bit string. To be noted here is that: the mute audio signal Ax has been scrambled and well dispersed in the emerging probability within each sample unit; however, from the view point of a longer bit string it repeats with a cycle of the original formula, therefore the probability of emergence appears particularly deviated in such a longer cycle.
FIG. 11 is a conceptual illustration of the dispersion of emergence in a congregated space. FIG. 11(a) shows the dispersion within each sample unit. The dispersion appears to have been well dispersed; however, because it is the same as the original formula itself, the space after scrambling is shrinking when viewed with a certain predetermined cycle, as shown in FIG. 11(b). In such a state, co-relationship before and after the scrambling can easily be detected, as a result, the method of scrambling may be readily read. In other words, the degree of scrambling is low.
In the practice, various efforts are being made to safeguard the scrambling method; for example, changing the initial value for each of the frames instead of merely using a formula, using a formula switching from time to time among a plurality of formulae, etc. However, if both of a known input data and the scrambled data observed are available, one can detect and read the scrambling using a computer at a comparative ease.
As described in the foregoing, there are following problems to be solved in the conventional method, in which the scrambling is conducted direct on a linear PCM audio data:
(a) Because the audio signal Ax is a linear PCM audio data, the probability of a certain specific data emerging with respect to a plaintext data in a very low level, or at mute, increases providing a particular deviation. Namely, the redundancy increases and the auto-correlation property will rise.
(b) Therefore, there is a high possibility for the scrambling to be detected; by knowing the state of the audio signal output Ax′ as a result of watching, and assuming the audio signal Ax′ is a mute data, and then picking up the scrambled audio signal Az′ out of the output of separation means 500 for comparison with output data.
(c) Especially in an AV system, where a compressed video data and a linear PCM audio share a common scrambling, the scrambling might well be detected first in the linear PCM audio data, whose degree of scrambling is low, through a co-relationship observed at mute using as a clue, and then in the compressed video data with a chain action, by making use of a method that has then become known.